This invention relates generally to an electrostatographic printing machine, and more particularly concerns a fusing apparatus employed therein.
In the process of electrostatographic printing, a latent image is recorded on a surface and rendered visible with particles. These particles are transferred to a sheet of support material in image configuration. Thereafter, the particles are permanently affixed to the sheet of support material forming thereon a copy of the original document. Electrostatographic printing includes both electrophotographic printing and electrographic printing. Generally, electrophotographic printing employs a photoconductive member which is charged to a substantially uniform level. A light image of an original document is projected onto the charged photoconductive member selectively dissipating the charge thereon. This records a latent image of the original document on the photoconductive member. Electrographic printing is distinguishable from electrophotographic printing in that it does not use a photoconductive member or a light image to create the latent image of the original document. However, in both of the foregoing processes, heat settable particles are generally employed to develop the latent image. After the particles are transferred to the sheet of support material, heat is applied thereto so as to permanently affix them to the sheet of support material.
Numerous techniques have been developed for applying heat to the particles transferred to the sheet of support material. One approach passes the sheet of support material, with the particles thereon, between a pair of opposed rollers. The outer surface of the heated fuser roll may be covered with polytetrafluoroethylene, commonly known as Teflon, to which a release agent such as a silicon is applied. This Teflon layer has a thickness of about several mils. The layer of oil applied thereto has a thickness of less than one micron. Silicon based oil possess a relatively low surface energy and is particularly advantageous for use with Teflon. A thin layer of this oil is applied to the surface of the heated roll and forms an interface between the roll surface and powder image on the sheet of support material. The low surface energy of this layer prevents the particles from transferring to the roll. Thus, the particles remain adhering to the sheet of support material. In this way, the particles are permanently affixed to the sheet of support material and do not smear or transfer to the fuser roll.
An alternative technique, employs a bare, heated metal roller. A low molecular weight polyethylene is applied, as a release agent, to this type of roller. The release agent is a liquid at the fuser roll operating temperature. In operation, a portion of the fuser roll continuously rotates through a sump containing the release material. In this manner, the release material is deposited on the surface of the fuser roll and a doctor blade, in contact therewith, regulates the thickness of the layer coated thereon. However, it has been found that during the standby mode, i.e., when the fuser roll is stationary, a thin layer of release material, which because of the wetting action, climbs the roller surface. At this point, because of oxidation and temperature, the polyethylene cross links and forms a bead (similar to a weld bead) of a brown substance on the heated bare metal surface. These beads will mar the copies being reproduced by the electrophotographic printing machine.
Accordingly, it is a primary object of the present invention to improve the fusing apparatus employed in an electrophotographic printing machine to prevent the formation of beads on the fuser roll employed therein.